Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation.
The present patent application is concerned primarily with Asymmetric DSL (ADSL) service, which allows data to be conveyed downstream to the subscriber at a rate of about 8 Mbit/s, and to be returned upstream from the subscriber at about 640 kbit/s. ADSL is based on a discrete multi-tone (DMT) transmission system, in which data are encoded using 256 different “tones,” each corresponding to a frequency band 4.3125 kHz wide. Recommendation G.992.1 (ex-G.dmt) of the International Telecommunication Union (ITU), which is incorporated herein by reference, specifies the physical layer characteristics of the ADSL interface to the subscriber line, including the allocation of the tones in the frequency spectrum to upstream and downstream service.
Annexes A and B of G.992.1 specify requirements for ADSL operation on telephone lines that are also used for “Plain Old Telephone Service” (POTS) or for ISDN (Integrated Services Digital Network) services. When operating over POTS, upstream ADSL service is allocated tones 6 through 31 (25.875 to 138 kHz). Downstream service may use all of tones 6 through 255 (25.875 up to 1104 kHz), as long as echo cancellation is applied in the upstream service range. Alternatively, downstream service may be limited to tones 32 through 255 if necessary to reduce near-end crosstalk (commonly referred to as “NEXT”) with the tones of the upstream signals. The range below about 25 kHz is left for POTS audio signals. On the other hand, when operating over ISDN, the range below 138 kHz (corresponding to tones 0-31) is unavailable to ADSL, as it must be left clear for ISDN transmission in a lower band. Tones 32 through 63 are allocated to ADSL upstream transmission, while tones 32 through 255 are allocated to ADSL downstream transmission. In practical implementations, however, it is frequently necessary to limit the downstream transmission to an upper band, roughly covering the range of tones 64-255, in order to reduce NEXT noise.
A range of variations on the standard ADSL spectral profiles have been proposed, in order to increase the upstream and/or downstream data rates while reducing crosstalk interference. One option for this purpose is to introduce a “notch” in the downstream power spectral density (PSD) mask. A scheme of this sort was proposed, for example, by Neulender in “A high-performance ADSL over POTS mask which is compatible with FDD ADSL over ISDN,” published as contribution TD-19 to the TM6 Standards Committee of the European Telecommunications Standards Institute (ETSI), February, 2000, which is incorporated herein by reference. The proposed mask allows both upstream and downstream ADSL service to use the lower frequency range (tones 6-31) when operating over POTS. The notch in the mask attenuates downstream signals in the range of tones 32-63, which are allocated only to upstream service in order to reduce the NEXT noise which disturbs the upstream signal of ADSL over ISDN service. The downstream signals above tone 64 are substantially unattenuated. When ADSL service is to be provided on an ISDN subscriber line, the lower range (tones 6-31) is simply switched out of both the upstream and the downstream ADSL PSD masks.
For efficient communications over a DSL channel (as well as other types of digital communication channels), it is necessary for the modems at either end of the channel to estimate and compensate for the channel characteristics. Typically, the modems automatically run through a standard start-up procedure for this purpose before beginning actual data communications. Each of the modems transmits a known training signal to the other. The receiving modem compares the received signal to the expected training signal in order to “learn” the channel characteristics. In ADSL modems, for example, the comparison is used to adaptively set the coefficients of digital equalization filters in order to compensate for distortion in the channel. Existing start-up procedures implicitly assume that the PSD mask applied to the signals is constant. Such procedures do not take account of the possibility that there might be an intentionally-induced notch, such as that proposed by Neulender.